(a) Field of the Invention
The present invention relates to a fuel cell vehicle, and more particularly, to a method and apparatus of increasing output efficiency of a fuel cell system by estimating an amount of water within the fuel cell system.
(b) Description of the Related Art
A fuel cell system is a type of power generation system which converts chemical energy of a fuel directly into electric energy within a fuel cell stack. A fuel cell vehicle using such a fuel cell system has advantages, such as reduction in exhaust gas and improvement in fuel efficiency, but in consideration of disadvantages of the fuel cell system, such as production of water and power performance, a fuel cell hybrid system having a power storage unit, i.e., another energy storage device differing from the fuel cell, is employed. A fuel cell hybrid vehicle may be equipped with a power storage unit, i.e., a high voltage battery or a super capacitor (supercap), as a separate power source for providing power necessary to drive a motor, in addition to a fuel cell as a main power source.
A fuel cell system mounted in the fuel cell hybrid vehicle includes a fuel cell stack to generate electric energy, a hydrogen supply device to supply hydrogen as a fuel to the fuel cell stack, an air (oxygen) supply device to supply oxygen in air as an oxidant necessary for an electrochemical reaction to the fuel cell stack, a thermal management system (TMS) which discharges heat of reaction of the fuel cell stack to the outside of the system, controls the driving temperature of the fuel cell stack and performs a water management function, and a fuel cell system controller to control the overall operation of the fuel cell system. Through such a configuration, the fuel cell system generates electricity by reaction between hydrogen serving as the fuel and oxygen in air, and discharges heat and water as by-products of the reaction.
A Proton Exchange Membrane Fuel Cell or Polymer Electrolyte Membrane Fuel Cell (PEMFC) has been identified for use in vehicles as a type of fuel cell having the highest power density, and the PEMFC has a short starting time and a short power conversion reaction time due to the low operating temperature thereof.
A fuel cell stack mounted in the PEMFC includes a Membrane Electrode Assembly (MEA) in which electrode/catalyst layers in which electrochemical reaction occurs are attached to both surfaces of a polymer electrolyte membrane to which hydrogen ions are moved, a Gas Diffusion Layer (GDL) serving to uniformly distribute reaction gases and to transmit generated electricity, gaskets and clamps to maintain air-tightness and proper clamping pressures of the reaction gases and cooling water, and a bipolar plate to move the reaction gases and the cooling water, and generates current by fuel cell reaction when hydrogen and oxygen (air) are supplied.
In the fuel cell stack, hydrogen is supplied to an anode (referred to as a “fuel electrode”), and oxygen (air) is supplied to a cathode (referred to as an “air electrode” or “oxygen electrode”).
Hydrogen supplied to the anode is separated into protons (H+) and electrons (e−) by the catalyst of the electrode layers formed on both surfaces of the electrolyte membrane and only the protons (H+) pass through the electrolyte membrane, i.e., a cation exchange membrane, and are transmitted to the cathode and, simultaneously, the electrons (e−) are transmitted to the cathode through the GDL and the bipolar plate, formed of a conductor.
In particular, in the cathode, protons (H+) supplied to the cathode through the electrolyte membrane and electrons (e−) transmitted to the cathode through the bipolar plate meet oxygen in air supplied to the cathode by the air supply device, thus producing water.
A flow of electrons (e−) through an external wire is generated according to movement of protons (H+) and such a flow of electrons (e−) generates current. Further, heat is subordinately generated during water production reaction.
Reactions at the electrodes of the PEMFC will be described, as follows.[Reaction at Anode] 2H2→4H++4e−[Reaction at Cathode] O2+4H++4e−→2H2O[Overall Reaction] 2H2+O2→2H2O+electric energy+thermal energy
In the above reaction, protons (H+) should pass through a polymer membrane, membrane permeability of protons (H+) is determined by a function of water content and, as the reaction progresses, water is produced and humidifies the reaction gases and the membrane.
If the reaction gases are dry, the entirety of water produced by the reaction is used to humidify air and thus the polymer membrane runs dry. Therefore, in order to properly drive a fuel cell, the polymer membrane should be kept humid. The reason for this is that membrane permeability of protons (H+) is determined by a function of water contained in the membrane.
On the other hand, if the membrane is excessively wet, pores of the Gas Diffusion Layer (hereinafter, referred to as “GDL”) are clogged and the reaction gases may not contact a catalyst. As a result, maintaining of a proper amount of water in the membrane is very important.
Therefore, in order to improve performance of a sulfonated fluoropolymer membrane serving to perform a function of exchanging protons (H+) in a polymer electrolyte membrane fuel cell, the membrane should contain a proper amount of water.
Although a fuel cell receives air in the atmosphere, instead of pure oxygen as an oxidant, air in the atmosphere is not generally humid enough to wet a membrane thoroughly. Therefore, the air must be sufficiently humidified so as to smoothly operate the fuel cell, prior to supply of the air in the atmosphere to the fuel cell.
Due to these characteristics, it is necessary to supply air containing a proper amount of water to the fuel cell, and in many fuel cell vehicles, air humidified by a humidification system is supplied to a stack. Further, a humidifier used in fuel cell vehicles is of a passive type which is continuously humidified by water (vapor) produced from chemical reaction in a stack.
However, an excessive amount of water (i.e., produced water) may occur within the humidifier during both general driving of a vehicle and in a constant current mode, known as a mode which is advantageous for humidification (a driving mode in which output current of a fuel cell is fixed to a constant value).
Accumulation of the excessive amount of water in the humidifier may cause problems, such as lowering of humidification efficiency, cell voltage sudden decrease during driving of a vehicle, blockage of an air flow path to a stack due to freezing of water within the humidifier in winter, and physical damage to the humidifier.
Therefore, a method of effectively controlling a fuel cell system in which the above-described problems are prevented by solving accumulation of an excessive amount of water in a humidifier while maintaining proper humidity of air of a fuel cell stack is required.